Conventionally, spherical tanks, independent prismatic tanks, membrane tanks, horizontal type cylindrical tanks, etc., are known as tanks to be installed on a ship that transports a liquefied gas. Examples of the liquefied gas stored in such a tank include liquefied petroleum gas (LPG: about −41° C.), liquefied ethylene gas (LEG: about −104° C.), liquefied natural gas (LNG: about −163° C.), and liquefied hydrogen (LH2: about −253° C.).
In recent years, for example, development of a LNG terminal that liquefies, on the ocean, natural gas excavated from an offshore gas field or the like and stores it on the ocean as liquefied natural gas (LNG) has been conducted. One example of such a LNG terminal is one with a ship hull shape and a fixed-point retention function. Such a LNG terminal includes equipment for lifting the natural gas from the seafloor, a liquefier for liquefying the natural gas into LNG, a power generating unit, a transfer apparatus, etc., which are disposed on the upper part of the hull. The hull includes storage equipment therein. Desirably, the storage equipment is a liquefied gas tank that allows the deck of the hull to have a flat plane so that the aforementioned equipment and apparatuses can be readily disposed on the deck of the hull, and allows a great amount of liquefied gas to be stored inside the hull.
In one example of prior art relating to a support structure of this kind of liquefied gas tank, a plurality of first members fixed to a surrounding structural body surrounding a prismatic semi-membrane tank are slidably engaged with a plurality of second members fixed to four side walls and a top wall of the tank (see Patent Literature 1, for example). One first member and one second member engaged therewith form a support assembly, and the support assembly includes an inclined surface that slopes downward toward the center of the tank. The inclined surface is a contact surface between the first member and the second member, and the first member and the second member are engaged with each other such that they can move relative to each other only along the inclined surface. That is, in this support structure, the second member is prevented from being spaced apart from the first member in both parallel and perpendicular directions to the side walls or the top wall of the tank.
In another example of prior art relating to a tank support structure, a plurality of coupling members connect between each side wall of a prismatic semi-membrane tank and a surrounding structural body (see Patent Literature 2, for example). Each coupling member extends horizontally, and one end of the coupling member is coupled to a side wall of the tank by a ball joint, and the other end of the coupling member is coupled to the surrounding structural body by a ball joint. With the coupling members, the support structure allows the side walls of the tank to make in-plane movement relative to the surrounding structural body.